The present invention relates to automatic exposure control systems for x-ray imaging apparatus; and more particularly to x-ray detectors utilized in such exposure control systems.
Conventional fluoroscopic x-ray imaging equipment include a source for projecting a beam of x-rays through an object being imaged, such as a medical patient. The portion of the beam which passes through the patient impinges upon an x-ray detector which converts the x-rays attenuated by the patient into photons which then are converted into an electric image signal. The x-ray detector may comprise an image intensifier tube and a video camera, or a combination of a scintillator in front of a two-dimensional photodetector array. The electrical image signal from either type of detector is processed to display an image of the patient on a video monitor.
The image signal also is applied to the input of a feedback loop which responds to the level of the image signal by producing a control signal which regulates the x-ray exposure and thus brightness of the image on the video monitor. An example of this type of feedback control is described in U.S. Pat. No. 4,573,183. The control signal regulates the biasing of the x-ray tube and the gain of the video camera. This feedback control loop ensures that the image produced in the video monitor has sufficient brightness and contrast for proper viewing by an x-ray technician or physician.